It is known in the art to provide fluid working machines in which the flow of working fluid into and out of a working chamber of cyclically varying volume is controlled on each cycle of working chamber volume by actively controlling the opening or closing of at least one electronically controlled valve, to select the net displacement of working fluid by the working chamber on each cycle of working chamber volume. This is known, for example, from EP 0361927 in which a low pressure valve which regulates the flow of working fluid between a working chamber and a low pressure manifold is actively controlled to enable a pump to carry out either an active cycle or an idle cycle. EP 0494236 developed this concept and introduced an actively controlled high pressure valve which regulates the flow of working fluid between a working chamber and a high pressure manifold, enabling a motor to carry out either an active cycle or an idle cycle and also enabling a fluid working machine to carry out either pumping or motoring cycles.
In each case, the LPV is actively controlled to select between active and idle cycles, and in some embodiments to control the fraction of maximum stroke volume which is displaced during active cycles. In order to enable active control, each valve has a respective solenoid which is coupled to a valve member.
The HPV is also typically actively controlled although in the case of a pump, the HPV can be operated in a solely passive way, for example, it may be a normally closed, pressure openable check valve. By active control we include the possibility of a valve being actively opened, actively closed, actively held open or actively held closed. An actively controlled valve may also move passively in some circumstances. For example, a LPV may be actively closed but open passively when the pressure in a cylinder drops below the pressure in the low pressure manifold.
Machines of this type have several advantages, including energy efficiency, an ability to rapidly respond to changes in demand, and a compact size. Although these machines have proven highly effective, to further develop them it would be advantageous to further simplify the control mechanism and to further reduce the bulk and complexity of the valve actuator mechanisms.
Accordingly, the invention seeks to provide a valve actuation mechanism which is simpler, smaller and/or more reliable than known valve actuation mechanisms for machines of this type.